Alkoxycarbene complexes of nickel(II)

Abstract Alkynylnickel complexes trans -C 6 Cl 5 Ni(PPhMe 2 ) 2 CCR (IIIa, R  H; IIIb, R  Me; IIIc, R  Et; IIId, R  CH 2 OH; IIIe, R  CH 2 CH 2 OH; IIIf, R  Ph; IIIg, R  C 6 H 4 OMe- p ) have been prepared from trans -[C 6 Cl 5 Ni-(PPhMe 2 ) 2 L]ClO 4 and monosubstituted acetylenes in the presence of triethylamine, and their reactions with alcohols in the presence of perchloric acid were studied. Complexes IIIa and IIIe afforded alkoxycarbene complexes trans -[C 6 Cl 5 Ni-(PPhMe 2 ) 2 {C(OR′)Me}]ClO 4 (IVa, R′  Me; IVb, R′  Et; IVc, R′  n-Pr) or trans -C 6 Cl 5 Ni(PPhMe 2 ) 2 { C(CH 2 ) 3 O }]ClO 4 (IVd), respectively, but IIIb either decomposed or afforded trans -C 6 Cl 5 Ni(PPhMe 2 ) 2 CHC(OMe)Me, depending on the amount of acid used. Treatment of IVaIVd with amines resulted in deprotonation to give α-alkoxyvinyl complexes, trans -C 6 Cl 5 Ni(PPhMe 2 ) 2 C(OR′)CH 2 (VIaVIc) or trans -C 6 Cl 5 Ni(PPhMe 2 ) 2 CCHCH 2 CH 2 O (VId), the reaction being reversible. A 1 H NMR study indicated: (i) that the carbene methyl and the vinyl protons IV or VI are D-exchangeable by MeOD without catalyst; (ii) that the basicity of VIa is comparable to those of amines; (iii) that the carbene complexes IVaIVc have two isomers due to hindered rotation about the C(carbene)O bond in solution, IVb existing in the Z -form in the solid state; (iv) that the rotationalbarriers (° G ≠ ) about the C(carbene)O bond in IVb and the NiC-(carbene) bond in IVd are 20 (or more) and 11.7 kcal/mol, respectively. These results are explained in term of double bond character of the carbene carbon and its surrounding atoms.